1. Field of the Invention
The present invention relates to a sheet conveying apparatus for conveying a sheet such as a recording sheet in an image forming apparatus such as a printer.
2. Description of the Related Art
In general, an image forming apparatus such as a printer has a conveying mechanism for conveying a sheet and a skew correcting mechanism for correcting skew of the sheet in a conveying direction.
The conveying mechanism includes a conveying roller rotated by a driving source and a pinching roller which is disposed opposite to the conveying roller to pinch the sheet and driven by the rotation of the conveying roller. The conveying mechanism conveys the sheet by rotating the conveying roller.
The skew correcting mechanism corrects the skew of the sheet in the conveying direction by abutting the front edge of the sheet against a registration roller or a registration shutter disposed upstream in the conveying direction of the sheet from an image forming section.
When skew correction is performed for a skewed sheet, the sheet is twisted between the skew correcting mechanism and the conveying mechanism, thereby generating a turning force M as shown in FIG. 7. This may cause skew again due to the turning force M after the skew correction of the sheet, which is to be sent to the image forming section.
As shown in FIG. 7, conveying rollers R1, R2, and R3 are supported by a rotating shaft C in a conventional image forming apparatus. The conveying rollers R1, R2, and R3 are attached to the rotating shaft C in a unified manner in the rotation direction around the rotating shaft C and are attached to the rotating shaft C movably in the axial direction of the rotating shaft C with coil springs S1 to S6 interposed between the conveying rollers R1, R2, and R3. In this image forming apparatus, skew of the sheet P is corrected by abutting the front edge of the sheet P in the conveying direction against the registration roller A. The image forming apparatus eliminates a thrust force F, which is generated by the turning force M caused by the twist (distortion) of the sheet P after the skew correction, by moving the conveying rollers R1, R2, and R3 in the axial direction of the rotating shaft C. Thus, the foregoing turning force M is eliminated, thereby solving the above problem.
Moreover, the coil springs S1 to S6 bias the conveying rollers R1, R2, and R3, which have been moved by the thrust force F, from axial both sides of the conveying rollers R1, R2, and R3, respectively. This enables the conveying rollers R1, R2, and R3 to return to predetermined positions, respectively, in some cases (Refer to Japanese Patent Application Laid-Open No. H02-018244).
Further, Japanese Patent Application Laid-Open No. H11-020993 proposes an arrangement for correcting skew of a sheet by measuring a skew amount of the sheet by using a sensor and varying a rotation amount of a pair of registration rollers disposed on both sides of the sheet in the width direction, which is perpendicular to the conveying direction of the sheet. Moreover, in this arrangement, besides the skew correction of the sheet, conveying rollers disposed upstream in the conveying direction of the sheet from the registration rollers are moved in an axial direction parallel to the width direction of the sheet.
The foregoing arrangement disclosed in Japanese Patent Application Laid-Open No. H02-018244, however, has a problem that a conveying path is not stable until the sheet abuts against the conveying rollers for performing skew correction of the sheet, since the plurality of conveying rollers are always movable in the axial direction of the conveying shaft.
Moreover, the conveying rollers is attached to the conveying shaft in a unified manner in the rotation direction around the conveying shaft in the arrangement disclosed in Japanese Patent Application Laid-Open No. H02-018244. Therefore, if a skew correction amount of the sheet is relatively large, the skew correction of the sheet may be insufficient to remove the twist of the sheet in some cases. Therefore, in order to achieve a sufficient effect, more or less it is necessary to slide the sheet toward the conveying rollers. On the other hand, however, there is a need to suppress sliding of the sheet as much as possible in order to obtain a sufficient conveying force to convey the sheet and it is difficult to satisfy both of the conditions incompatible with each other.
Therefore, it is difficult to prevent skew of the sheet while reliably conveying the sheet and therefore it has conventionally been a cause of a difference in force between the left and right sides in the width direction of the sheet and an uneven conveying distance. Thus, particularly a high-resolution ink jet recording apparatus has a problem that color unevenness and density unevenness occur in an image recorded on the sheet.
Further, in the arrangement disclosed in Japanese Patent Application Laid-Open No. H11-020993, there is a need to provide a plurality of drive mechanisms in order to drive the sensor for detecting skew of the sheet and the plurality of conveying rollers independently of each other, which leads to a problem of an increase in manufacturing cost and upsizing of the apparatus.